U.S. patent number 10,506,136 [Application Number 14/854,932] was granted by the patent office on 2019-12-10 for printer calibration using limited range reflection scanners as input sources.
This patent grant is currently assigned to KODAK ALARIS INC.. The grantee listed for this patent is Kodak Alaris Inc.. Invention is credited to Stuart Gerard Evans, Steven R. Schmidt, Kevin Craig Scott.
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United States Patent |
10,506,136 |
Scott , et al. |
December 10, 2019 |
Printer calibration using limited range reflection scanners as
input sources
Abstract
A method of calibrating a printer using a reflective scanner is
disclosed. Because the reflective scanner used for calibration may
only be able to accurately measure a limited density range that is
less than the full density range of the printer, some information
from the reflective scanner is disregarded or deemphasized during
the calibration process. A calibration page is printed and scanned.
Lookup tables (LUTs) that comprise the printer calibration values
are updated based on adjustments calculated from the scanner for
density regions where the scanner produces relatively accurate
measurements, but updated based on the preexisting settings for
density regions where the scanner produces relatively inaccurate
measurements. In transitions regions between accurate and
inaccurate regions, the LUTs are adjusted based on a combination of
measurements from the scanner and the preexisting settings.
Inventors: |
Scott; Kevin Craig (Rochester,
NY), Evans; Stuart Gerard (Rochester, NY), Schmidt;
Steven R. (Rochester, NY) |
Applicant: |
Name |
City |
State |
Country |
Type |
Kodak Alaris Inc. |
Rochester |
NY |
US |
|
|
Assignee: |
KODAK ALARIS INC. (Rochester,
NY)
|
Family
ID: |
52667703 |
Appl.
No.: |
14/854,932 |
Filed: |
September 15, 2015 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20160072983 A1 |
Mar 10, 2016 |
|
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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14460819 |
Aug 15, 2014 |
9213923 |
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61867302 |
Aug 19, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G06K
15/027 (20130101); H04N 1/6044 (20130101); H04N
1/60 (20130101); G06K 15/1878 (20130101); G06K
15/40 (20130101); H04N 1/6033 (20130101); G06K
15/4015 (20130101); H04N 2201/0091 (20130101) |
Current International
Class: |
H04N
1/60 (20060101); G06K 15/00 (20060101); G06K
15/02 (20060101) |
Field of
Search: |
;358/406,504,518
;399/49,72 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Rust; Eric A.
Attorney, Agent or Firm: Hogan Lovells US LLP
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This application is a continuation of U.S. patent application Ser.
No. 14/460,819, files Aug. 15, 2014, which claims the benefit of
U.S. Provisional Application No. 61/867,302, filed Aug. 19, 2013,
which is hereby incorporated by reference in their entirety.
Claims
The invention claimed is:
1. A method of characterizing a limited range reflective color
scanner for use as a calibration input source, comprising: a)
selecting a type and model of scanner to be characterized; b) using
a measurement instrument to read a test target comprising a
plurality of printed patches; c) using the scanner to read the
plurality of printed patches on the test target; d) converting scan
data obtained from the scanner reading the plurality of printed
patches on the test target to a form compatible with data obtained
from the measurement instrument; e) using a processor to compare
the converted scan data with the data obtained from the measurement
instrument; f) using the processor to calculate a weighting from
the comparison for use by a calibration algorithm; g) using the
calibration algorithm to update a lookup table, wherein the update
weighs more heavily scan data that corresponds to accurate ranges
for the selected type and model of the scanner and disregards scan
data that corresponds to inaccurate ranges for the selected type
and model of the scanner; and h) modifying printing parameters
using the updated lookup table.
2. The method of claim 1 wherein the printing parameters are used
to optimize a printed output of a printer.
3. The method of claim 1 wherein selecting the type and model of
scanner to be characterized comprises: selecting multiple scanners
to be characterized; and averaging scan data from the multiple
scanners.
4. The method of claim 1 wherein scan data from the scanner is
converted to Status A Densities, DIN Densities, or Channel
Independent Densities based on the associated densitometer or
spectrophotometer readings.
5. The method of claim 1 wherein the accurate and inaccurate ranges
for the selected type and model of scanner are limited by flare,
platen glass contamination, electronic noise, clipping, or a
voltage offset on a CCD.
6. The method of claim 1 wherein the accurate ranges for the
selected type and model of the scanner is smaller than an output
range of a chosen printer.
7. The method of claim 1 wherein the modified printing parameters
are configured for printer types comprising electrophotographic,
thermal dye diffusion, inkjet, or digital photographic printer
types.
8. The method of claim 1 wherein the updated lookup table and
modified printing parameters are used to produce a printed output
that is calibrated in critical locations of human eye
sensitivity.
9. The method of claim 8 wherein the printed output is provided by
a printer separate from the scanner.
Description
FIELD OF THE INVENTION
This invention relates generally to the field of printer
calibration and more particularly to the process of using a
reflection scanner characterized by model or type to perform
printer calibrations.
BACKGROUND OF THE INVENTION
Printers, especially printers used to produce continuous tone or
photographic images, require routine calibration to compensate for
the use of different print media lots and types or the use of
different inks, toners, donor ribbons, types and lots. Calibration
also addresses printer electronics and components whose operating
characteristics drift over time due to wear and usage.
Performing tone scale calibration for a printer requires that a
plurality of printed patches on a target be measured by some means.
The measurements are then processed through a calibration
algorithm, which generates new printing parameters, such as a
lookup table (LUT), to optimize the printed output. These
measurements are usually made by an instrument that measures the
reflective density, such as a densitometer or a spectrophotometer.
Typically, the units of measurement are Status A density, which is
a measure of the amount and or combinations of dyes or pigments
present in a given patch. The calibration instrument's density
measurement range is typically greater than the printer's own Drain
to Dmax density range. This greater range is desirable and required
for most existing calibration methods, as the calibration
instrument's measurements can be used to accurately and optimally
calibrate the printer through its entire Dmin to Dmax density
range.
However, there are several drawbacks to using these instruments for
printer calibration. First, densitometers and spectrophotometers
are expensive. They also require calibration themselves, require
knowledgeable users and are ancillary equipment not used to produce
prints or to scan hardcopy media digitalization or duplication.
Lastly, densitometers and spectrophotometers use factory provided
calibration targets, which are also expensive, and can be lost,
damaged or degraded if they are improperly handled or stored. It is
therefore desirable to be able to effectively use a less costly
measurement device for printer calibration.
Reflective scanners, such as a flat-bed print scanner, can be
utilized for this purpose and are readily available. However, these
devices typically have a density measurement range that is smaller
than that of the printer's output range and are not designed to
produce a stable, invariant response across their entire response
range. Reflective scanners measurements drift due to changes in
lamp output, changes in electrical components, debris such as
pollen and dust and film caused by off-gassing from plastic
components within the scanner housing that collects on the
underside of the scanner platen glass. In addition to variations
due to drift over time and usage, scanners of this type vary
between manufacturers and within productions lots.
It is known to use reflection scanners as input sources for printer
calibration; however these techniques all have requirements that
limit their accuracy and applicability.
U.S. Pat. No. 8,203,768 teaches a calibration method that includes
scanning a test patch, which comprises a plurality of halftone
cells, to obtain reflectance values, calculating subset averages of
reflectance values as defined by an averaging window, and
calculating an overall average based on the subset averages. This
calculation pertains exclusively to halftone printing systems and
integrates the halftone dot patterns to generate a reflectance
value. The densitometers or spectrophotometers used in traditional
printer calibration include an aperture that is typically around 5
mm in diameter, and the reflected light that passes through that
aperture is optically averaged by the device. Reflection scanner
based printer calibration for halftone images involves averaging
values in some region analogous to the aperture of a
spectrophotometer or densitometer.
U.S. Pat. No. 7,719,716 describes techniques for using a scanner to
calibrate printers and requires that reflectance value be
calculated for each patch on a test target. This method would
preclude using test targets with patches that are within the gamut
of the printer, but outside the accurate gamut of the scanner.
U.S. Pat. No. 7,319,545 assumes the scanner is a relatively stable
measurement device and will remain in a state that is sufficiently
close to its intended design such that it does not need to be
characterized. The disclosure assumes that the drift associated
with the printer will be much greater than the variability
associated with the scanner. However, in reality, density
measurement deviations for reflective scanners can be large in
certain density regions, especially on worst case scanner
types.
U.S. Pat. No. 6,909,814 describes converting data from an object
scanner and then correcting that data whenever the object scanner
response does not correspond to that of a standard scanner
response. Every object scanner must be so characterized. Every
object scanner must have a reference to compare it to the results
of a standard scanner and this scanner calibration has to be done
from time to time. It is impractical to correct every scanner on a
routine basis.
U.S. Pat. No. 6,671,067 requires that a factory produced reference
target and a printed target be scanned simultaneously, referred to
as a combined target. As previously discussed, factory provided
calibration targets are expensive, can be lost, and can fade or be
damaged if improperly handled or stored.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a typical system for practicing the present
invention;
FIG. 2 illustrates a typical process for practicing the present
invention;
FIG. 3 is an overview flow diagram for calibrating a printer
according to an embodiment of the present invention;
FIG. 4 is a flow diagram depicting the method for characterizing a
scanner according to an embodiment of the present invention;
FIG. 5a is a graph depicting the response for a typical scanner
that is out of tolerance;
FIG. 5b is a graph depicting the response for a typical scanner
that is near tolerance;
FIG. 6 is a flow diagram of the method for calibrating a printer
according to an embodiment of the present invention;
FIG. 7a is a graph depicting the sigmoid function used to establish
the operational and non-operational ranges of the scanner
response;
FIG. 7b is a formula used to create the sigmoid function graph;
FIG. 7c is a formula used to adjust the slope position of the
sigmoid function;
FIG. 8 is a graph depicting a sample calibration adjustment
weighting level vs. density based on the sigmoid function; and
FIG. 9 is a graph depicting a sample density measurement showing
error from the actual response.
DETAILED DESCRIPTION OF THE INVENTION
Performing tone scale calibration for a printer usually requires
that a plurality of printed patches on a target be measured by some
means. The measurements are then processed through a calibration
algorithm, which generates new printing parameters, such as a
lookup table (LUT), to optimize the printed output. These
measurements are usually made by an instrument which measures the
reflective density, such as a densitometer or a spectrophotometer.
Typically, the units of measurement are Status A density, which is
a measure of the amount and or combinations of dyes or pigments
present in a given patch. The instrument's density measurement
range is typically greater than the printer's own Dmin to Dmax
density range. This is desirable and required for most existing
calibration methods, as the instrument's measurements can be used
to accurately and optimally calibrate the printer through its
entire Dmin to Dmax density range. Such reflective measuring
instruments are typically costly. It is desirable to be able to
effectively use a less costly device to make the measurements.
Reflective scanners, such as a flat-bed print scanner, can be
utilized for this purpose and are readily available; however, these
devices typically have a density measurement range that is smaller
than that of the printer's output range.
There are a variety of reasons for a reflective scanner's limited
range. At the mid-to-high density end, flare, platen glass
contamination, electronic noise or a voltage offset on the CCD
input can cause the density readings to be different than the
actual print density, as measured with a more accurate instrument.
At the low density end, a particular reflective scanner may not be
able to accurately measure down to the printer's Dmin, clipping
many of the low density patches to a code value 255. Thus, when
using a reflective scanner as a calibration input source, typical
calibration algorithms would be unable to accurately calibrate the
printer's entire output density range.
This invention diminishes this problem when a limited range
reflective scanner is present, and no specialized calibration
instrument is available. The printer can be calibrated using target
patch measurements from a reflective scanner, producing better
printed results than if no instrumented calibration were performed
at all. A novel calibration algorithm can allow for compromises
outside of the characterized reflective scanner's density
measurement range. The invention is described in detail with
particular reference to certain preferred embodiments thereof, but
it will be understood that variations and modifications can be
effected within the spirit and scope of the invention.
FIG. 1 shows a typical system for practicing the present invention.
FIG. 2 is a simplified illustration of the overall process of this
invention, where in a printer renders a calibration target which is
then scanned.
FIG. 3 is an overview flow diagram for calibrating a printer
according to an embodiment of the present invention. The
calibration algorithm takes three inputs, which are density aims,
patch densities derived from scanner code values and the initial
received LUT. The calibration algorithm generates a new LUT. A
final, corrected LUT is generated by combining weighted
combinations of the initial received LUT and the new LUT. The
weighting of the new LUT versus the initial LUT is determined by
the non-operational and operational ranges of the scanner.
FIG. 4 is a flow diagram depicting the method for characterizing a
scanner according to an embodiment of the present invention. First,
a type and model of scanner to be characterized is selected.
Second, a densitometer or spectrophotometer is used to read a test
target. Third, the same test target is read by the scanner (or
scanners) to be characterized. If multiple scanners are to be
characterized, the results from the scanners may be averaged. The
scan data is then converted and compared with the data from the
densitometer or spectrophotometer. Lastly, a weighting is
calculated from this comparison.
FIG. 5a is a graph depicting the response for a typical scanner
that is out of tolerance. FIG. 5b is a graph depicting the response
for a typical scanner that is near tolerance.
FIG. 6 is a flow diagram of the method for calibrating a printer
according to an embodiment of the present invention. The method
begins with an initial LUT, scanner characteristic data (such as
that generated by the process illustrated in FIG. 4) and density
aim values. The first step in the method is printing a calibration
target with the printer to be calibrated. Then, the calibration
target is scanned with the scanner and the scan data is converted.
Next, the calibration algorithm processes the converted scan data
with the density aim values and provides output to the weighting
step in the form of a new LUT. The weighting step operates on the
new LUT and the initial LUT to create an corrected LUT. If the
corrected LUT indicates that calibration is achieved, the process
ends. Alternatively, if the corrected LUT indicates that
calibration is not achieved, the process iterates (i.e., a new
calibration target is printed and the steps are performed
again);
In an embodiment of the present invention, a particular reflective
scanner's range limitations are first characterized, using a target
with known patch densities. This scanner range characterization is
stored electronically in the printing system. The calibration
algorithm then uses this characterization to diminish the
calibration algorithm's applied adjustment. The applied adjustment
will be tapered in some mathematical fashion, as the measured patch
density range falls outside of the reflective scanner's accurate
density measuring range. The measurement limitations of the scanner
are due to various identifiable causes, allowing us to disregard
the scanner information in a prorated manner in these regions. The
accurate information can still be used to calibrate the printer's
output to a better state than would be the case if no calibration
were performed at all, while the prorating causes no visible
discontinuity artifacts. Extrapolation may be used to fill-in the
density regions, which are not accurately measured by the scanner.
This will be done by mathematically "blending" the print scanner's
density measurements, at the extremes of its accuracy range, with
the printer's factory default calibration position at these
locations, with the object being to remove any discontinuities from
the resulting calibration. The resulting printed output will be
"mostly" calibrated in the critical locations of human eye
sensitivity within the print scanner's accurate density range, and
will taper off to the printer's factory default calibration outside
of this range. While not ideal, this approach results in printed
output that is better than if no instrumented calibration were
performed at all. FIG. 7a is a graph depicting the sigmoid function
used to establish the operational and non-operational ranges of the
scanner response for use with the weighting operation. FIG. 7b
shows a formula that may be used to create the sigmoid function
graph. FIG. 7c is a formula used to adjust the slope position of
the sigmoid function. FIG. 8 is a graph depicting a sample
calibration adjustment weighting level versus density based on the
sigmoid function. FIG. 9 is a graph depicting a sample density
measurement showing error from the actual response.
The invention may use a profile, created offline, which calculates
Status A density from the scanner's reported RGB code values. The
profile is created from the response of a "typical" scanner (i.e.
center-of-population). Further, the "scanner characterization"
necessary to the invention, may include Finding a "worst case"
scanner, of a particular model or type of scanner; where "worst
case" is defined as that which deviates the most from actual Status
A density, in one or more defined regions of the density
measurement range such as between Status A densities of 1.0 and
2.5, and between Status A densities of 0.01 and 0.06. The "worst
case" scanner may have excessive contamination on the underside of
the glass, or other defect causing the deviation between actual and
reported Status A densities.
The invention does not attempt to correct every portion of the
printer's density range with the information from the scanner.
Rather, for a particular type of scanner the inventive method may
use the scanner data in certain density regions where that data
corresponds to accurate ranges of that scanner type to generate a
new or corrected printer LUT. Scanner data that is in certain
density regions that correspond to the inaccurate ranges of that
scanner are disregarded in favor of the initial LUT. The invention
prorates the scanner data in certain density regions where that
data corresponds to the transitional ranges of that scanner type
(that occur between regions of relative accuracy and relative
inaccuracy), using a portion of each of the new or corrected LUT
and the initial LUT. The invention derives the accurate,
transitional and inaccurate ranges from a particular "worst case"
references scanner.
In some embodiments of this invention, color reflection scanners
are characterized by model and manufacturer. In some embodiments,
the color printer types include electro-photographic, thermal dye
diffusion, inkjet, and digital photographic printers. Some
embodiments of this invention may use measurement units such as
Status A Densities. DIN Densities, or channel independent
densities. In some embodiments, the chosen printer LUTs include the
reference or current LUT that was used to print the calibration
target, a reference LUT that corresponds to the factory defaults,
and a new LUT that is calculated from the measurement units
obtained from the scanner.
* * * * *